1. Field of the Invention
The present invention relates to an injection control system of an internal combustion engine for performing an injection quantity learning operation.
2. Description of Related Art
As a method of inhibiting generation of combustion noise and nitrogen oxides in a diesel engine, a method of performing a pilot injection for injecting a very small quantity of fuel before a main injection is known. Since a command value of the pilot injection quantity is small, improvement of accuracy of the small quantity injection is necessary to sufficiently exert the effects of the pilot injection of inhibiting the generation of the combustion noise and the nitrogen oxides. Therefore, an injection quantity learning operation for measuring a deviation between the command injection quantity of the pilot injection and a quantity of actually injected fuel (an actual injection quantity) and for correcting the injection quantity on a software side is necessary.
A fuel injection control system disclosed in Japanese Patent Application No. 2003-185633 can perform the injection quantity learning operation highly accurately. The control system performs a single injection from an injector into a specific cylinder of an engine when the engine is in a no-injection state, in which a command injection quantity outputted to the injector is zero or under. The engine is brought to the no-injection state if fuel supply is cut when a position of a shift lever is changed or when a vehicle is decelerated, for instance. The control system calculates an actual injection quantity based on a variation of an engine rotation speed caused by the single injection. If an error is generated between the actual injection quantity and the command injection quantity of the pilot injection, the control system corrects the command injection quantity in accordance with the error.
Usually, the command injection quantity is corrected by calculating an injection period correction value from a characteristic shown in FIG. 8 based on the difference between the actual injection quantity measured by performing the single injection and the command injection quantity. In FIG. 8, ΔT represents the correction value of the injection period, ΔN is the variation in the operating state of the engine (an engine state variation ΔN), and Ntrg is a target value of the engine state variation ΔN. For instance, the engine state variation ΔN is a variation (an increase) in the rotation speed of the engine caused by the single injection. This characteristic shown in FIG. 8 aims to shorten a period necessary to complete the correction by increasing the correction value ΔT as the deviation between the command injection quantity and the actual injection quantity increases. The engine state variation ΔN corresponds to the actual injection quantity and the target value Ntrg corresponds to the command injection quantity. However, it takes a much longer time to find the correction value ΔT for compensating for the deviation in the case where the actual injection quantity largely deviates from the command injection quantity along a decreasing direction than in the case where the actual injection quantity deviates along an increasing direction, as explained below.
Characteristics of an injector of a diesel engine are shown in FIG. 9. In FIG. 9, Q represents the actual injection quantity, Qc is the command injection quantity, and TQ is the injection period. If the actual injection quantity Q largely deviates along the decreasing direction from a solid line q1 to a broken line q2 shown in FIG. 9, a no-injection range, in which the actual injection quantity Q is zero, is enlarged from a range A1 to a range A2 shown in FIG. 9. Meanwhile, a characteristic of the engine state variation ΔN changes from a solid line n1 to a broken line n2 shown in FIG. 9. At that time, if a first injection is performed based on a first injection pulse width TQ1 shown in FIG. 9, the injector injects no fuel and a variation of the engine rotation speed (the engine state variation ΔN) due to the injection is not generated. In this state, a value provided by subtracting the actual injection quantity Q from the command injection quantity Qc coincides with the command injection quantity Qc, since the actual injection quantity Q is zero. In such a case, if the injection period correction value ΔT is calculated by the above method, a value “a” shown in FIG. 8 or 9 is calculated as the injection period correction value ΔT.
If a second single injection is performed based on an injection pulse width TQ2 shown in FIG. 9, in which the correction value “a” is reflected, no fuel is injected. Accordingly, the correction value remains “a”.
Thus, in the case where the actual injection quantity Q deviates largely along the decreasing direction and the actual injection quantity Q provided after the correction remains zero, the constant correction value is calculated regardless of the degree of the deviation of the characteristic of the injector. Therefore, the effect of shortening the period necessary to complete the correction by increasing the correction value as the deviation increases cannot be achieved. As a result, the correction takes a long time.
If the actual injection quantity Q deviates largely along the increasing direction from the command injection quantity Qc, the single injection quantity injected for the injection quantity learning operation will increase excessively. If the injection is continued at the command injection quantity, noise will be generated and emission will be deteriorated.